1. Field Of the Invention
The present invention relates to a knock control system for an internal combustion engine for detecting a knock generated in the engine and controlling knock control factors, such as, an ignition timing and an air-fuel ratio of the mixture gas to be supplied to the engine, depending on the detected knock condition in the engine.
2. Description of the Prior Art
A knock control system for an internal combustion engine has been proposed as disclosed in such as U.S. Pat. No. 4,993,387. In system, a knock criterion level or a knock decision level V.sub.KD is derived based on a standard deviation .sigma. of a substantially logarithmic conversion value of a knock intensity value V detected by a knock sensor. Specifically, the knock decision level V.sub.KD is derived by an equation of V.sub.KD =S.sup.3 .cndot.V.sub.M, wherein S represents a value corresponding to the standard deviation .sigma. of the normal distribution of the substantially logarithmic conversion value (hereinafter also referred to as "log(V)") of the knock intensity value V, and V.sub.M represents a value corresponding to a median value of the log(V) normal distribution.
The system determines in every routine cycle whether the detected knock intensity value V is between V.sub.M /S and V.sub.M (V.sub.M /S.ltoreq.V.ltoreq.V.sub.M) and updates the value S such that a probability of satisfying V.sub.M /S.ltoreq.V.ltoreq.V.sub.M becomes one third (about 33%). This means that the value S corresponds to 1.sigma., i.e. .sigma..apprxeq.log(S), wherein .sigma. represents the standard deviation of the log(V) normal distribution. The system further updates the value V.sub.M based on a decision whether the detected knock intensity value V is above or below the value V.sub.M to converge the knock intensity values V greater than the value V.sub.M and those less than the value V.sub.M to 50%, respectively, so that the value V.sub.M continues to correspond to the media value of the log(V) normal distribution.
As a result, the conventional system can absorb differences in characteristics of individual knock sensors used and in engine types, time-dependent variation in characteristics of the used engine and knock sensor, and variation in engine speed and load, which respectively affect characteristics of the knock sensor signals V, by deriving the knock decision level V.sub.KD based on the value S (representing the standard deviation .sigma. of the log(V) normal distribution) and the value V.sub.M (representing the media value of the log(V) normal distribution) which are sequentially updated using the detected knock sensor signals V. Accordingly, the system is capable of providing a reliable knock detection without being adversely affected by the above-noted difference and variation.
However, when a knock sound happens to become extremely large or a knock is concentrated to a particular cylinder of a multi-cylinder engine, the distribution of the knock intensity value V is largely disturbed or disordered due to knock-related components included in the knock sensor signal so that the value S is caused to be increased. As a result, the knock decision level V.sub.KD is also increased to induce more engine knocking, which, in turn, further increases the value S to further increase the knock decision level V.sub.KD, leading to divergence of the value S and the knock decision level V.sub.KD.
The grounds for the divergence of the value S will be explained hereinbelow with reference to FIGS. 12, 13 and 14.
As shown in FIG. 12, the distribution of log(V) becomes normal distribution which is symmetrical with respect to the media value log(V.sub.M) when no knock-related components are included in the knock sensor signal V. On the other hand, when a knock is generated, large magnitudes of the knock intensity values V reflect on the log(V) distribution so that the right end of the log(V) distribution deviate from the normal distribution, as shown in FIG. 13. In general, as the generated knock gets larger, the disturbance on the right side of the log(V) distribution also gets larger, while the disturbance on the left side of the distribution does not occur as often under the normal engine knocking. However, when a magnitude of the knock happens to increase abruptly or when the knock is concentrated to a particular cylinder, i.e. the knock is not equally distributed among the cylinders, it is possible that the left side of the distribution is also disturbed, as shown in FIG. 14.
Since the foregoing conventional system updates the value S by maintaining the probability of V.sub.M /S.ltoreq.V.ltoreq.V.sub.M at one third (about 33%), when the left side of the log(V) distribution is disturbed as shown in FIG. 14, the value S corresponding to the standard deviation .sigma. of the log(V) distribution is also necessarily increased to realize a cumulative relative frequency of log(V.sub.M /S).ltoreq.log(V).ltoreq.log(V.sub.M) in the log(V) distribution as being one third (about 33%). In other words, the value S is necessarily increased since the standard deviation .sigma. of the log(V) distribution is increased (.sigma..apprxeq.log(S)).